(1) Field of the Invention
The present invention relates to a structure of a universal joint applicable to a steering system of an automotive vehicle.
(2) Background of the Art
A universal joint is a joint used to link a pair of shafts at predetermined a angle and to transmit rotational torque from one shaft to the other, used, for example, in a steering system of an automotive vehicle.
An automotive steering system includes a steering shaft rotatably supported at a predetermined tilt angle in a vehicle compartment, a steering gear box disposed at a front portion of the vehicle body, and a universal joint with a low frictional coefficient intervened between intermediate shafts.
The universal joint includes a pair of yoke portions which face each other and rotatably link the intermediate shafts via a trunnion.
A link portion of the universal joint linking the pair of yoke portions and trunnion includes a hole penetrated into the pair of yoke portions, a bearing cup fitted into the hole, a spider of the trunnion intervened with a needle bearing and inserted into the bearing cup, and a seal ring which houses a lubricant and is closely contacted against the bearing cup and trunnion. A circular projection having a predetermined area is projected from the outside of the center of the bearing cup toward its inside. A rough surface having a convex portion and a recessed portion is formed on the projection. The rough surface is contacted against an end surface of the spider with a lubricant such as grease intervened on the rough surface of the projection. A stopper is formed on the pair of yokes and fixed to the bearing cup. Thus, swing torque (friction) between the trunnion and yokes can relatively be reduced.
However, in the previously proposed structure of a universal joint described above, trouble occurs such that steering force is heavy making a steering wheel difficult to return to an original state when applied to a vehicle in which an auxiliary output such as a hydraulic system is used to relieve the steering force, i.e., a so-called power-assisted steering system.
This is because the swing torque between the trunnion and the pair of yoke portions is relatively large. Hence, more improvement is required for the universal joint structure.